1. Field of the Invention
This invention is related to plasma-arc torches which are generally used for cutting, welding and spray bonding and more specifically to an electrode having improved pilot arcing for such torches.
2. Description of the Invention
Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding and spray bonding of workpieces and operate by directing a plasma consisting of ionized gas particles toward a workpiece. In the operation of typical plasma-arc torches, such as illustrated in U.S. Pat. Nos. 4,585,921; 4,581,516; 4,324,971; 4,170,727; and 3,813,510; assigned to the same assignee as the present invention, a gas to be ionized is supplied to the front end of the torch and flows between a pair of electrodes before exiting through an orifice in the torch tip. The electrode which is at a relatively negative potential is usually referred to as the "cathode" or simply as the "electrode." The torch tip which is adjacent to the end of the electrode, at the front end of the torch, constitutes the relatively positive potential electrode or "anode." When a sufficiently high voltage is applied, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. A pilot pulsating voltage between the electrode and the torch tip maintains an arc known as the pilot, or non-transferred arc. The ionized gas in the gap is blown out of the torch ad appears as a flame that extends externally off the tip. During this transferred arc operation, the workpiece serves as the anode. As the torch head or front end is brought down towards the workpiece, the arc jumps or transfers between the electrode and the workpiece instead since the impedance of the workpiece current path is lower than the impedance of the torch tip current path.
In conventional torches, the consumable negative potential electrode or cathode is usually made of copper and commonly has at its end a refractory metal insert such as one from the Group IVb elements of the Periodic Table. Conventional electrodes generally assume the form of a smooth elongated copper rod with a hafnium or zirconium insert recessed into its end.
In starting a plasma-arc torch, a high-frequency, high-voltage pulse(s) causes an arc to jump between the tip and electrode. This happens when the electric potential applied across the tip and the electrode produces a local electric field that exceeds the dielectric-breakdown field strength of the gas therebetween. This breakdown is then followed by the DC pilot arc.
For a given potential, this local electric field is a function of both the distance between, and the relative geometries of the torch tip and the electrode. Generally, a higher electric field is obtained with a closer spacing or with a smaller radius of curvature of the electrode surface.
On the other hand, the dielectric-breakdown field strength of the plasma forming gas is generally a function of the type of gas used, as well as its pressure and temperature. Various gases or mixtures of gases may be used, for example air.
In practice, the electric potential required for arcing is not a well defined value. It assumes a standard distribution caused by random molecular gas motion, cathode spot wandering on the metal surfaces with consequent localized pitting and oxide formation, and mechanical tolerances over production runs.
Typcially, in the case of the smooth electrodes, any one torch and electrode combination has a short term variation of 4 KV. Such torch may most probably arc at 8 KV but there are instances when it will arc at as low as 5 KV or as high as 12 KV. That is, the pilot voltage required assumes a standard distribution that peaks at 8 KV with the tails extending to 5 KV and 12 KV. When the pilot voltage of a torch is designed in the 8 KV to 8.5 KV range (i.e. in the most probable range), there will still be quite a number of firings which will produce no arcing. Test results show that the smooth electrode fires less than half the time because most of the required arcing voltages are above what the power supply provides. Experience shows an annoying tendency to go for long periods of time with no piloting following by short bursts of piloting. This combination gives erratic, unacceptable operation.
One solution is to upgrade the power supply to generate arc voltage above 12 KV to cover the worst case. This should give reliable piloting. While this is possible, the practical constraints are increased size, increased cost, increased radio frequency interference, and the need to better insulate the torch and leads.
Alternatively, the required firing voltage can be lowered by either reducing the dielectric-breakdown field strength of the gas or increasing the local electric field strength.
The reduction of the dielectric-breakdown field strength can be achieved by such means as reduced gas pressure. However, the gas pressure, flow, and swirl are already selected for best cutting performance and cannot be changed.
The local electric field strength can be increased by closer spacing, or designing pointed instead of smooth surfaces.
Close spacing brings mechanical tolerance problems and the spacing is also optimized for best cutting performance.
Pointed surfaces are difficult to make, can wear down, and may not be conducive to the controlled gas flow that gives best cutting performance.